It consists of the pianist going to the piano, and not hitting any keys for four minutes and thirty-three seconds. (He uses a stopwatch to time this.) In other words, the entire piece consists of silences -- silences of different lengths, they say.
On the one hand, as a musical piece, 4'33" leaves almost no room for the pianist's interpretation: as long as he watches the stopwatch, he can't play it too fast or too slow; he can't hit the wrong keys; he can't play it too loud, or too melodramatically, or too subduedly.
On the other hand, what you hear when you listen to 4'33" is more a matter of chance than with any other piece of music -- nothing of what you hear is anything the composer wrote.
john_cage_4m33s.mid(39 bytes, MIDI)
433.ogg(8,827 bytes, Ogg Vorbis)
433.au.gz(2,982 bytes -- 3MB unGZ'd, AU)
433.zip(3,108 bytes -- 3MB unzipped, WAV)